1. Field
The technology of the present invention relates to radio frequency antennas and, and more specifically to radio frequency antennas with a power feed configuration that reduces the specific absorption rate (SAR) of the wireless device.
2. Background
Portable, wireless devices are prevalent in today's society. A person can hardly leave their home without using or encountering the use of a portable, wireless device. For purposes of this application, a portable, wireless device should be construed broadly and includes, but is not limited to, cellular telephones, handheld computers, email devices (such as a BLACKBERRY®), MPEG devices and MP3 players, electronic games, PDAs, and the like.
With the prevalence of wireless devices, many countries are concerned about radio frequency energy or electromagnetic energy being absorbed by a human. SAR is a measurement of the amount of radio frequency energy (radiation) is absorbed by the body when using a radio transmitter such as a cellular telephone. While most electronic devices emit some type of radiation, cellular telephones are of particular interest due to the proximately of the transmitter to the human head during use. For example, referring to FIG. 1, a conventional cellular telephone 100 being used by a user 150 is provided. Cellular telephone 100 includes a printed circuit board (PCB) 102 having radio frequency electronic circuits thereon, not specifically shown, and is coupled to battery 104. Cellular telephone 100 includes an antenna housing 106 containing a radiator 108, shown as a meander trace, but could be other conventional antennas such as, for example, a whip antenna, a helical antenna, or the like. Radio frequency power is provided to radiator 108 from PCB 102 via a feed line 110 and contact 112, which is typically a spring contact, but could be a solder, a press fit, or the like connection. Feed line extends from PCB 102 to radiator 108 along housing 106. Feed line 110 may be a meander line feed or the like. Thus, feed line is, as will be explained in connection with the technology of the present application, relatively close to the head of user 150. While the technology of the present application is specifically discussed with reference to cellular telephones, one of ordinary skill in the art will recognize on reading the disclosure that the technology could be used in any electronic device, including the portable, wireless devices described above.
Currently, the Federal Communication Commission in the United States requires cellular telephones to have a SAR level of about 1.6 watts per kilogram of body tissue (1.6 W/kg) or less. Other countries have similar limits, for example, the European limit for SAR is about 2 W/kg.
Many cellular telephones and other devices include electromagnetic shielding to reduce the amount of radiation emitted by the cellular telephone. While shielding is effective to reduce SAR, it increases the cost, weight, and size of the cellular telephone or other portable, wireless device. Thus, additional mechanisms are desired to further reduce the SAR.